Biol 108 - Topic 16 (Gymnosperms)

Seed plants originated around 360 million years ago (mya) and are the _____ in terrestrial ecosystems.

dominant primary producers

Shared derived traits of seed plants

1. Reduced gametophytes:

2. Heterospory

3. Ovules

4. Pollen

5. Seeds

Reduced gametophytes: shared derived of seed plants

gametophytes of seed plants develop within walls of spores that are retained within tissues of the parent sporophyte.

• reduction of gametophyte size + indep

Heterospory:  shared derived of seed plants

All seed plants are heterosporous; they produce two sizes of spores: megaspores and microspores.

• Spores are not dispersed; they are retained within the sporophyte.

• Microsporangia produce microspores, giving rise to male gametophytes.

• Megasporangia produce megaspores, giving rise to female gametophytes.

Ovules: shared derived of seed plants

an ovule consists of a megasporangium (2n), megaspore (1n), and one or more protective integuments (2n; gymnosperm ovules have one integument, angiosperm ovules usually have two integuments).

• Ovules develop into seeds after fertilization.

• The megasporangium (2n) produces a megaspore (1n) via meiosis, enclosed in protective integuments (2n).

• Megaspore is not dispersed; pollination triggers female (mega)gametophyte (1n) development.

• Female gametophytes have several thousand cells in gymnosperms, but are reduced to a few nuclei and cells (embryo sac) in angiosperms.

Pollen: shared derived of seed plants

microspores develop into male gametophytes within pollen grains (coated with tough outer wall of sporopollenin to withstand desiccation, UV, and physical damage).

• Male gametophytes are transported in pollen grains to the ovules;

• fertilization is done without the external release of sperm (fertilization without water). - sperm released in ovule

• Microsporangium (2n) produces microspores (1n) via meiosis.

• Microspores are not dispersed; they develop into pollen grains.

• Microspores undergo mitosis to produce tiny male gametophytes (1n) that have at minimum a generative cell and a tube cell that become sperm nuclei and a pollen tube, respectively, after pollination.

Seeds: shared derived of seed plants

a seed develops from a fertilized ovule and consists of an embryo (next-generation sporophyte), food supply, and outer seed coat.

Pollination transports ___ to the ___ for fertilization

pollen; ovule

In seedless plants, single-celled, ____ require ____ to swim thro (usu short distances)

flagellated sperm; water

In seed plants, the entire male gametophyte is carried inside the ________, eliminating the need for ___ during fertilization.

pollen grain; water

Pollen can be dispersed over great distances by _____

wind or animals

If a pollen grain germinates, it creates a ____ that discharges sperm nuclei (without flagella) into the female gametophyte within the ovule.

pollen tube

Seed dispersal:

The seed is the dispersal stage for seed plants. cf. non-seed plants where spores are the dispersal stage

Seeds provide evolutionary advantages over spores:

1. Seed plants evolved structures to enhance seed dispersal by air, water, or in/on animals (seeds are larger than spores, so they are not as easily dispersed by wind).

2. Seed may be transported long distances by wind, water, or animals

3. Seeds have external coats that protect the embryo.

   1. Seeds remain dormant until conditions are favourable for germination

4. Seeds have an energy-dense supply of stored food to support early seedling growth:

   1. large-seed plants produce few seeds, but with a lot of stored food, they can germinate and grow in low light (e.g. forest understory);

   2. small-seed plants produce many seeds, but successful germination requires optimal conditions.

Evolution of gymnosperms:

• The first seed plants appear in the fossil record ~360 mya; the earliest fossil evidence of extant gymnosperms is ~305 mya.

• Gymnosperms dominated Mesozoic (251-65 mya) terrestrial ecosystems because they were better adapted than seedless vascular plants to the drier conditions of the Mesozoic (i.e. fertilization without water; dispersal via seeds).

  • Gymnosperms got terrestrial features like seeds, pollen, and cuticles (for protection)

• Angiosperms began to replace gymnosperms near the end of the Mesozoic, and angiosperms now dominate most terrestrial ecosystems.

Extant gymnosperms consist of four phyla:

• Cycadophyta (~350 species): Cycads were diverse and dominant in the Mesozoic, but are now limited to small populations in the tropics and subtropics (many species endangered). Cycads grow very slowly, producing large palm-like leaves on short, unbranched stems. Cycads produce large cones on separate male and female sporophytes that are insect-pollinated. Cycads (& ginkgos) are unique among seed plants in having flagellated sperm.

• Gingkophyta (one living species, Ginkgo biloba).

• Gnetophyta (~75 species in three genera).

• Coniferophyta (~600 species): Most conifers are woody shrubs or trees.

  • Wind-pollinated, but slow reproduction.

    • Extant conifer species are found primarily in cool to cold environments; dominant trees in boreal and alpine regions in Canada (~35 species of conifers).

    • Most conifers are evergreens and can carry out photosynthesis year-round.

    • Adapted to cold, dry habitats (xylem is resistant to collapse during freezing; leaves with thick waxy cuticle; reduced leaf surface area; leaf retention).

The life cycle of gymnosperms

• Key features of the gymnosperm life cycle are:

  1. Gymnosperms have dominant sporophyte generations; the mature tree is the sporophyte (2n).

  2. Gymnosperms develop seeds from fertilized ovules. Ovules support female gametophytes that are protected by the sporophyte.

  3. Transfer of male gametophytes (sperm) to ovules by pollen. Chemically resistant pollen cell wall (sporopollenin outer layer) protects the male gametophyte during dispersal, and sperm is not released into the environment.

The life cycle of a conifer (Pinus):

• The pine tree is a sporophyte (2n) that produces sporangia on scale-like leaves (sporophylls) clustered in cones.

• Conifers are heterosporous, with megasporangia and microsporangia in female and male cones (on the same or different tree).

• Small cones produce microspores that develop into male (micro)gametophytes enclosed in pollen grains.

• Familiar larger cones contain ovules, which produce megaspores that develop into female (mega)gametophytes.

• Pollination by wind: pollen grains of many conifers have air bladders.

• Seed dispersal by wind.

• Fertilization and seed development in conifers are slow.

Seed

contains embryo + nutrients surrounded by a protective coat for long-distance dispersal

Contrast w/ bryophytes and gymnosperms for gametophytes

Bryophytes - gametophytes were dominant

Seedless VS seed plants

All seed plants are heterosporous

Microspores develop into _____, which contain _____

pollen grains; male gametophytes

Contrast w/ bryophytes and gymnosperms for sperm release

Bryophytes release flagellated sperm into environ, gymnosperms don’t release sperm into environ (transported in pollen)

Gymnosperm “naked seeds”

• seeds exposed on sporophylls (specialized leaf type) that forms cones

• 1st seed plants were progymnosperms (fern-like leaces, woody stems, and seef-like strucs

• Most gymnosperms are cone-bearing plants called conifers

Angiosperms

• seeds enclosed in fruit (mature ovaries)

Gymnosperms are a _____

monophyletic group

Cycadophyta phylum

Cycads were diverse and dominant in the Mesozoic, but are now limited to small populations in the tropics and subtropics (many species endangered).

• Cycads grow very slowly, producing large palm-like leaves on short, unbranched stems.

• Cycads produce large cones on separate male and female sporophytes that are insect-pollinated.

• unlike more seed plants, Cycads (& ginkgos) are unique among seed plants in having flagellated sperm.

Gingkophyta phylum

• have flagellated sperm like cycads (ancestral trait retained)

  • Pollen that has flagellated sperm is dispersed by wind

• single living species (Ginkgo biloba)

• Separate male + female sphotophyte trees

Phylum Gnetophyta (~75 species in three genera)

• 3 genera: Gnetum, Ephydra, and Welwitschia

• Phylym grouping based on molecular evidence

Conifers were widespread before evolution of angiosperms.

True or False?

True